Sure, slavers are jerks, and Candie is sort of violent and crazy, but he's still a rich white guy. And what do rich white guys love? More money. As horrible as it sounds, Candie simply thinks of Broomhilda as property. We're never given any reason to believe otherwise. If Django offered him more than Broomhilda was worth, he would have absolutely no incentive to refuse. Big money for a slave who can't even kill things with her bare hands? Candie would have been doing racist back flips all the way to the bank.
The Weinstein Company
"Now I can buy enough slaves to fill THREE torture holes!"
But let's suppose Candie likes being petty to strangers more than he likes money. Why would he have to find out that Django is Broomhilda's husband? Just because honesty is the best policy? In fact, why the hell does Django have to be there at all? Schultz goes into Candie's home pretending to be Django's boss anyway. Why couldn't he have just gone in alone and explained that he heard there was a slave who spoke German here (this was their excuse for being interested in Broomhilda in the actual movie, remember) and that he could use help translating some of the finer nuances of his enemy's pleas for mercy?
For what it's worth, Tarantino says that Schultz came up with the obviously flawed, needlessly convoluted plan because he gets off on scheming and deceit. But come on -- we haven't seen such transparent ass-covering since that time we got thrown out of the community pool for our Saran Wrap bathing suit.